MessageBox returning instantly after a MFC DoModal() call

Question

The topic says it all: I'm having a problem where MessageBox returns immediately (without displaying anything) if it is being called after a CDialog::DoModal(). I've tried all kinds of MessageBox: MessageBoxA, MessageBoxW, CWnd::MessageBox (by using the dialog), AfxMessageBox. None works and they return 1 immediately. I tried saving the HWND of the dialog and using that as the first parameter. I tried passing GetDesktopWindow() as the HWND parameter. I tried stuff such as MB_YESNO|MB_ICONSTOP. Nothing worked.

I suspect I'm missing something really obvious but for the life of me, I can't figure out what and I've been looking for a solution for over 2 hours now. I tried by creating a new project using the MFC Wizard, selecting the dialog template and simply adding a call to MessageBoxA right after the DoModal() call:

CdelmeDlg dlg;
m_pMainWnd = &dlg;
INT_PTR nResponse = dlg.DoModal();
MessageBoxA(0, "test", "test", 0);

The dialog displays just fine, but when I click Ok or Cancel, the message box simply does not show up.

If I place the MessageBoxA() call above the DoModal() call, it works perfectly.

Edit: is there any way to prevent this behavior?

does the following work in your context? MessageBoxA(0, "test", "test", MB_TASKMODAL); — ZagNut
I had already tried that without success. Turns out the problem was the fact that the thread was being terminated by MFC when the dialog window was closed, just like Daniel Mošmondor posted. Removing the line that assigns dlg to m_pMainWnd fixed my issue. — Birt

Daniel Mošmondor Daniel Mošmondor · Accepted Answer · 2011-12-28T00:24:15

From memory:

MFC framework is designed in a way that if main window (in your case dlg) exits, message loops is over and there will be no more of anything that you can do GUI-wise.

More info (from the Creators):

The Microsoft Foundation Class Library will automatically terminate your thread when the window referred to by m_pMainWnd is closed.

from:

http://msdn.microsoft.com/en-us/library/f3ddxzww(v=vs.80).aspx

MessageBox returning instantly after a MFC DoModal() call

1 Answers